Updates to Debian Preseed configuration

- when 'secret' directory is defined, by default new Debian servers will
  have random password. It will be recorded in the 'secrets/' directory of
  the Debian Preseed server (apt_cache_host). In that case password change
  won't be forced;

- by default install 'sudo' and 'lsb-release' packages needed for
  succesful playbook run on new hosts;

- postinst.sh script run at the end of the installation process will
  automatically create admin user based on the username of the account
  from which Ansible is run, and add its ~/.ssh/id_rsa.pub key to root and
  admin account. This way after succesful ssh login to new host, main
  playbook can be run immediately without the need to run
  'playbooks/newhost.yml' first;